Searching for oil or, more generally, hydrocarbons has become more demanding in terms of hardware and devices in recent years because oil and gas fields or reservoirs are located deeper under the earth's surface or in places difficult to reach below the sea bed. Prospecting for an exploitation of hydrocarbon fields demands hardware which is more resistant to environmental challenges such as higher loads and corrosion, which were less important in the past.
Modern joints are generally designed with metal to metal seals made by the contact between two surfaces, positioned usually either at one end or at both ends of the threaded portion of the joint. The two contact surfaces reciprocally interfere in the elastic range of the modulus of elasticity for an appropriate stress magnitude. However in specific situations, resilient seals are needed instead of or in combination with metallic seals to prevent penetration of external fluids in the interstices of the threads.
It is therefore a design requirement that the joint seals resist penetration of the external fluids, or at least do not allow continuous exchange of fluids that have already penetrated the joint with the surrounding fluids, in order to reduce corrosion rate of the joint.
To solve the mentioned problems various solutions have been suggested, one of which consists of inserting a seal ring made of polymeric material, e.g. an O-ring, at the extremity of the joint near the threaded portion.
It is a widespread practice to use O-rings pre-mounted onto the box, but this practice has an important disadvantage: the seal rings can be damaged during mounting of the ring and/or during make-up.
As such seals are usually pre-mounted onto the box, when making up the box the seal is dragged together with the box over the pin's threads. Due to the diametrical interferences needed between pin, box and seal dimensions for the seals to work properly, along with the geometric parameters of the connection such as thread profile, pitch, taper angle, stab angle, etc, the seals are inevitably forced to crawl over the pin's threads, causing damage to the lower surfaces of the seals or permanent deformations to the seal which would jeopardize the sealing efficiency during operation.
Seal rings having more complicated shape have also been proposed, e.g. rings with an “H” or “U”-shaped cross section to increase efficiency and prevent sea water from penetrating into the gaps of the joint. However such seal rings are subject to such stresses and adverse environmental conditions, both during the initial make-up operation and during operational life, that their efficiency is much reduced after some time. Even though pre-mounting on the pin would be less harmful for the seal ring, some field applications require seal rings of simpler shape which must be pre-mounted onto the box.
U.S. Pat. No. 6,550,822 discloses a threaded joint including a water exclusion seal which is a pressure energized elastomeric seal positioned in a groove near each end of the joint to make a radial seal between the unthreaded area at each end of the joint and the unthreaded cylindrical area adjacent each end of the tubular section.
The shape of the ring section does not facilitate its dragging over the thread during make up. Because of this shape, the sharp corners hit the thread crests and are prone to get stuck in the thread. In case of joint break-out it becomes even more likely to damage the seal ring.
In case of failure, the portion of the pin with the vanishing thread remains unprotected against possible external corrosive fluids, since dope used to lubricate the joint is easily washed away by the infiltrating external fluid.
This joint uses standard seal rings available on the market. Therefore, the same ring design must be used under different connections, and does not make it easy to customise the ring geometry for each pin taper, stab angle, thread height and pitch, etc.
The cavity located between the lips renders the housing fragile, making it prone to rupture during break-out of the joint, when the lower part of the housing is dragged over the pin's threads, producing such a high deformation as to break it.
There have been many attempts to design improved configurations for seal rings to be housed in the vicinity of the nose of the box, but all are insufficient to withstand the higher requirements set for current joints.